(a) Field of the Invention
The present invention relates to a field emission cold cathode and a method for manufacturing the same and, more particularly, to the structure of a resistance layer serially connected with the emitter in a field emission cold cathode.
(b) Description of the Related Art
In general, a field emission cold cathode comprises a conical emitter having a pointed tip and a gate electrode having a submicron-order opening for providing a high electric field around the tip of the conical emitter for emitting electrons from the tip in the vacuum.
In the conventional field emission cold cathode, the distance between the emitter and gate electrode is small so that there sometimes occur a short-circuit failure between the emitter and gate electrode caused by a meltdown of the emitter due to a large current flowing through the emitter and triggered by the gaseous ambient of the emission. To prevent such a breakdown failure, it is proposed to provide a serial resistance layer to the emitter so as to limit the emitter current for prevention of the emitter meltdown.
Among the proposals to provide the resistance layer, a first conventional example is described in JP-A-5(1993)-36345, wherein the serial resistance layer is epitaxially grown for a silicon emitter. FIGS. 1A to 1F consecutively show a fabrication process for the first conventional example in sectional views thereof. In the fabrication process, a resistance layer 42 is formed by an epitaxial process as a lightly doped epitaxial layer on an N-type silicon substrate 41 which is connected to a cathode electrode. Subsequently, a heavily doped epitaxial layer 43 is formed thereon, followed by forming an oxide layer 44 on the epitaxial layer 43. Then, the oxide layer is patterned to form a mask pattern 44, followed by an isotropic dry etching of the heavily doped layer 43 and the resistance layer 42 by using the mask pattern 44 to form a protrusion from the heavily doped layer 43, as shown in FIG. 1B. Thereafter, a thermal oxidation step is effected to form a thermal oxide layer 45 and to sharpen the tip of the protrusion including the resistance layer 43 and heavily doped layer 42, as shown in FIG. 1C.
Next, electron beam evaporation step is effected to consecutively deposit an insulator film 46 and a gate electrode layer 47 from above to the entire surface of the wafer in the vertical direction, as shown in FIG. 1D. Then, an etching step is effected by using a hydrofluoric acid to remove the mask pattern 44 and insulator film 46, thereby selectively removing the gate electrode film 47 by a lift-off method in the vicinity of the emitter, i.e., emitter area. The etching step also removes the exposed oxide film 45 on the emitter to expose the conical emitter 48 including the heavily doped layer 43 and underlying serial resistance layer 42, as shown in FIG. 1E. A subsequent patterning step for the gate electrode layer 47 provides the structure as shown in FIG. 1F. The serial resistance layer 42 is associated with the heavily doped layer 43 to function as a protective layer for prevention of the emitter meltdown by alleviating the electric field around the tip of the conical emitter 48.
Among the proposals to provide the resistance layer, a second conventional example is described in JP-A-7(1995)-94076, wherein an emitter formed as a vacuum-deposited metal layer is provided with a patterned resistance layer. FIG. 2 shows the second conventional example which comprises an insulating substrate 51, a cathode layer 52 selectively formed on the substrate 51 to form a plurality of conductor pieces each connected to a cathode electrode not shown, a resistance layer 53 divided into a plurality of resistance sections each connected to the conductor pieces of the cathode layer 52, an insulator layer 54 overlying the resistance layer 53 and having a plurality of holes therein, a gate electrode layer 55 formed on the insulator layer 54 and having an opening corresponding to each hole, and a conical emitter 58 made of a metallic film and formed on the resistance layer 53 in the corresponding one of the holes in the insulator layer 54. The edge of the resistance layer section 53 is of a comb-shape having teeth connected to corresponding conductor piece of the cathode layer 52. Each resistance layer section 53 mounts thereon a group of emitters 58 (or emitter block) for protection.
In the second conventional example, if the emitter 58 and gate electrode layer 55 are short-circuited, the corresponding resistance layer 53 mounting the emitter 58 is fused at the edge portion thereof, i.e., at the teeth of the comb, to be disconnected from the corresponding cathode layer 52, thereby disabling the emitter block mounted on the resistance layer section 53 and including the short-circuited emitter 58. Although the emitter block itself does not operate thereafter, other emitter blocks can operate as usual to substantially maintain the function of the field emission cold cathode as a whole.
In the second conventional example, the comb-shape resistance layer section 53 connected to the cathode conductor layer 52 should have long and thin teeth in order to effectively break the connection between the resistance layer section 53 and the cathode layer 52 by fusing the teeth. That is, the resistance layer section 53 should have a sufficient space between two adjacent emitter blocks for the teeth. In order to decrease the occupied area for the field emission cold cathode, therefore, the number of emitter blocks should be minimum. However, the small number of emitter blocks involves a large area of the emitter block and accordingly, a large defective area caused by one defective emitter, thereby involving a trade-off between the small occupied area and a small defective area caused by one defective emitter.
Moreover, in the second conventional example, a sufficient high serial resistance is not obtained by the resistance layer section 53 because the resistance layer section 53 functions as a two-dimensional resistor, and even if a relatively high resistance is achieved after fabrication thereof, the resistance cannot be maintained after application of an excessive high voltage because of the small effective length of the resistance layer section 53. After all, substantially only the teeth of the resistance layer function as effective resistance portions.
On the other hand, in the first conventional example, since the resistance layer is formed as a part of the conical emitter, the resistance layer functions as the resistor in the thickness direction of the resistance layer. In this configuration, the thickness of the resistance layer is on the order of several tenths of micron at most since the emitter itself has a height of several microns. When a voltage on the order of 100 volts is applied between the gate electrode and an emitter, an electric field as high as 10.sup.5 volts/cm is applied in the resistance layer. The resistance layer reduces the resistance thereof due to the avalanche effect in this electric field range so that the resistance of the resistance layer is not stable in this range. An additional resistance layer, even if provided as an underlying layer for the conical resistance layer, does not effectively increase the serial resistance for the emitter because of the larger horizontal area of the additional resistance layer.